1. Field of the Invention
The present invention relates to a soldering flux used, for example, for soldering connections of circuit parts and the like to circuit boards provided with suitable metal wiring.
2. Description of the Related Art
Soldering of circuit parts on metal wiring sections of circuit boards requires subsequent removal of solder and metal oxides, etc. on the surface of the boards and prevention of reoxidation of the metal surface during soldering, while it is also necessary to lower the surface tension of the solder in order to accomplish satisfactory soldering. Soldering flux is used for this purpose.
A flux used for this purpose has conventionally incorporated inorganic acids as activators and particularly dibasic acids with relatively low molecular weights (for example molecular weights of 250 and less) have been employed as activators. However, when a synthetic resin is used as the base material, such as in recently used non-washing flux which requires no washing, or even when a rosin is used, when the proportion of rosin is low as with low-residue flux, the organic acid metal salt fails to totally dissolve in the resin and precipitates out in beads on the board. The organic acid in this precipitate is readily decomposed by moisture, becoming a cause of corrosion and insulation defects.
As a result, such non-washing flux is used with a minimal content of the dibasic acid or by replacing the dibasic acid with a monobasic acid. However, although such measures result in improvement from the standpoint of precipitation of the metal salt, the activating power is reduced, and this reduction in the activating power results in many soldering defects.
Conventional fluxes have employed rosins as the base resins However, when a residue remains on the board as in the case of non-washing flux, consequent results are cracking at low temperatures and peeling of the residue by vibrations, etc., thus leading to poor reliability with increased moisture penetration. Therefore, a synthetic resin is sometimes used either alone or in combination with the rosin in the non-washing flux. Yet, although the synthetic resin gives an improved quality to the residue film compared to the rosin alone, cracking and peeling still occur upon repeated temperature cycle tests and impact tests.